General Considerations in Machine Design

Following are the general considerations in designing a machine component :
1. Type of load and stresses caused by the load. The load, on a machine component, may act in several ways due to which the internal stresses are set up. The various types of load and stresses are discussed in chapters 4 and 5.
2. Motion of the parts or kinematics of the machine. The successful  operation of any machine depends largely upon the simplest arrangement of the parts which will give the motion required. 

The motion of the parts may be :
(a) Rectilinear motion which includes unidirectional and reciprocating motions.
(b) Curvilinear motion which includes rotary, oscillatory and simple harmonic.
(c) Constant velocity.
(d) Constant or variable acceleration.
3. Selection of materials. It is essential that a designer should have a thorough knowledge of the properties of the materials and their behaviour under working conditions. Some of the important characteristics of materials are : strength, durability, flexibility, weight, resistance to heat and corrosion, ability to cast, welded or  hardened, machinability, electrical conductivity, etc.
4. Form and size of the parts. The form and size are based on judgement. The smallest practicable cross-section may be used, but it may be checked that the stresses induced in the designed cross-section are reasonably safe. In order to design any machine part for form and size, it is necessary to know the forces which the part must sustain. It is also important to anticipate any suddenly applied or impact load which may cause failure.
5. Frictional resistance and lubrication. There is always a loss of power due to frictional resistance and it should be noted that the friction of starting is higher than that of running friction. It is, therefore, essential that a careful attention must be given to the matter of lubrication of all surfaces which move in contact with others, whether in rotating, sliding, or rolling bearings.
6. Convenient and economical features. In designing, the operating features of the machine should be carefully studied. The starting, controlling and stopping levers should be located on the basis of convenient handling. The adjustment for wear must be provided employing the various takeup devices and arranging them so that the alignment of parts is preserved. If parts are to be changed for different products or replaced on account of wear or breakage, easy access should be provided and the necessity of removing other parts to accomplish this should be avoided if possible.
The economical operation of a machine which is to be used for production, or for the processing of material should be studied, in order to learn whether it has the maximum capacity consistent with the production of good work.
7. Use of standard parts. The use of standard parts is closely related to cost, because the cost of standard
or stock parts is only a fraction of the cost of similar parts made to order. The standard or stock parts should be used whenever possible ; parts for which patterns are already in existence such as gears, pulleys and bearings and parts which may be selected from regular shop stock such as screws, nuts and pins. Bolts and
studs should be as few as possible to avoid the delay caused by changing drills, reamers and taps and also to
decrease the number of wrenches required. 8. Safety of operation. Some machines are dangerous to operate, especially those which are speeded up to insure production at a maximum rate. Therefore, any moving part of a machine which is within the zone of a worker is considered an accident hazard and may be the cause of an injury. It is, therefore, necessary that a designer should always provide safety devices for the safety of the
operator. The safety appliances should in no way interfere with operation of the machine.
9. Workshop facilities. A design engineer should be familiar with the limitations of his employer’s workshop, in order to avoid the necessity of having work done in some other workshop. It is sometimes necessary to plan and supervise the workshop operations and to draft methods for casting, handling and machining special parts.
10. Number of machines to be manufactured. The number of articles or machines to be manufactured affects the design in a number of ways. The engineering and shop costs which are called fixed charges or overhead expenses are distributed over the number of articles to be manufactured. If only a few articles are to be made, extra expenses are not justified unless the machine is large or of some special design. An order calling for small number of the product will not permit any undue expense in the workshop processes, so that the designer should restrict his specification to standard parts as much as possible.
11. Cost of construction. The cost of construction of an article is the most important consideration involved in design. In some cases, it is quite possible that the high cost of an article may immediately bar it from further considerations. If an article has been invented and tests of hand made samples have shown that it has commercial value, it is then possible to justify the expenditure of a considerable sum of money in the design and development of automatic machines to produce the article, especially if it can be sold in large numbers. The aim of design engineer under all conditions, should be to reduce the manufacturing cost to the minimum.
12. Assembling. Every machine or structure must be assembled as a unit before it can function. Large units must often be assembled in the shop, tested and then taken to be transported to their place of service. The final location of any machine is important and the design engineer must anticipate the exact location and the local facilities for erection.






A TEXTBOOK OF Machine Design
(S.I. UNITS)
[A Textbook for the Students of B.E. / B.Tech.,
U.P.S.C. (Engg. Services); Section ‘B’ of A.M.I.E. (I)]
2005
EURASIA PUBLISHING HOUSE (PVT.) LTD.
RAM NAGAR, NEW DELHI-110 055




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  • Communication of designs

    The most essential design activity, therefore, is the production of a final description of the artefact. This has to be in a form that is understandable to those who will make the artefact. For this reason, the most widely-used form of communication is the drawing. For a simple artefact, such as a door-handle, one drawing would probably be enough, but for a larger more complicated artefact such as a whole building the number of drawings may well run into hundreds, and for the most complex artefacts, such as chemical process plants, aeroplanes or major bridges, then thousands of drawings may be necessary.
    These drawings will range from rather general descriptions (such as plans, elevations and general arrangement drawings) that give an 'overview' of the artefact, to the most specific (such as sections and details) that give precise instructions on how the artefact is to be made. Because they have to communicate precise instructions, with minimal likelihood of misunderstanding, all the drawings are themselves subject to agreed rules, codes  and conventions.
    These codes cover aspects such as how to lay out on one drawing the different views of an artefact relative to each other, how to indicate different kinds of material, and how to specify dimensions. Learning to read and to make these drawings is an important part of design education.
    The drawings will often contain annotations of additional information. Dimensions are one such kind of  annotation. Written instructions may also be added to the drawings, such as notes on the materials to be used (as in Figure 1).

    Other kinds of specifications as well as drawings may also be required. For example, the designer is often required to produce lists of all the separate components and parts that will make up the complete artefact, and an accurate count of the numbers of each component to be used. Written specifications of the standards of workmanship or quality of manufacture may also be necessary.
    Sometimes, an artefact is so complex, or so unusual, that the designer makes a complete three-dimensional mock-up or prototype version in order to communicate the design. However, there is no doubt that drawings are the most useful form of communication of the description of an artefact that has yet to be made. Drawings are very good at conveying an understanding of what the final artefact has to be like, and that understanding is essential to the person who has to make the artefact. 
    Nowadays it is not always a person who makes the artefact; some artefacts are made by machines that have no direct human operator. These machines might be fairly sophisticated robots, or just simpler numerically-controlled tools such as lathes or milling machines. In these cases, therefore, the final specification of a design prior to manufacture might not be in the form of drawings but in theform of a string of digits stored on a disk, or in computer software that controls the machine's actions. It is therefore possible to have a design process in which no final communication drawings are made, but the ultimate purpose of the design process remains the communication of proposals for a new artefact.

    Engineering Design Methods
    Strategies for Product Design
    THIRD EDITION
    Nigel Cross
    The Open University, Mi/ton Keynes, UK
    JOHN WILEY & SONS, LTD
    Chichester- New York. Weinheim • Brisbane. Singapore. Toronto



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  • Classifications of Machine Design

    The machine design may be classified as follows :
    1. Adaptive design. In most cases, the designer’s work is concerned with adaptation of existing designs. This type of design needs no special  knowledge or skill and can be attempted by designers of ordinary technical training. The designer only makes minor alternation or modification in the existing designs of the product.

    2. Development design. This type of design needs considerable scientific training and design ability in order to modify the existing designs into a new idea by adopting a new material or different method of manufacture. In this case, though the designer starts from the existing design, but the final product may differ quite markedly from the original product.

    3. New design. This type of design needs lot of research, technical ability and creative thinking. Only those designers who have personal qualities of a sufficiently high order can take up the work of a new design.
    The designs, depending upon the methods used, may be classified as follows :
    (a) Rational design. This type of design depends upon mathematical formulae of principle of
    mechanics.
    (b) Empirical design. This type of design depends upon empirical formulae based on the practice and past experience.
    (c) Industrial design. This type of design depends upon the production aspects to manufacture any machine component in the industry.
    (d) Optimum design. It is the best design for the given objective function under the specified constraints. It may be achieved by minimising the undesirable effects.
    (e) System design. It is the design of any complex mechanical system like a motor car.
    (f) Element design. It is the design of any element of the mechanical system like piston, crankshaft, connecting rod, etc.
    (g) Computer aided design. This type of design depends upon the use of computer systems to assist in the creation, modification, analysis and optimisation of a design.






    A TEXTBOOK OF Machine Design
    (S.I. UNITS)
    [A Textbook for the Students of B.E. / B.Tech.,
    U.P.S.C. (Engg. Services); Section ‘B’ of A.M.I.E. (I)]
    2005
    EURASIA PUBLISHING HOUSE (PVT.) LTD.
    RAM NAGAR, NEW DELHI-110 055





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  • Computer-Aided Design and Manufacture of Injection Forging

    The design activity is responsible not only for the performance and appearance of the product but also for the cost of the component. Design, therefore, cannot be an isolated activity but must address all available manufacturing routes, with a view to optimizing the quality and cost of the component. With reference to nett-forming, the design exercise is conducted not only to specify the component-form but also to address all manufacturing constraints—machine, material, tooling, and processing conditions. 

    Computer-aided “design for manufacture” is currently the main form of implementing of the “concurrent
    engineering.” To enable this, CAD/CAM is popularly used as a design approach. Using CAD/CAM
    approaches, simultaneous design would be effected efficiently by supporting the designer with information
    on all possible resources required for the design and manufacture of components. Some CAD/CAM
    systems [42] have demonstrated the potential for the development into decision-support systems for
    component/tool design.
    Computer-aided design and manufacture for nett-forming by injection forging is being developed as
    an aspect of research associated with the development of a decision-support system [64].

    Methodology 
    In order to develop a decision-support system for component/tool design using a CAD/CAM approach, several design/evaluation methods have been developed [58, 60, 64–68]. These are described briefly in the following texts.

    Geometric Modeling
    The popular strategy used for the development of the design-support systems for forging was to evolve
    a 2D-CAD system for component and tool design. The system was linked to a knowledge-based system
    to enable the evaluation of manufacturability. Subsequent to the evaluation of the geometry, the component
    was transferred to a CAD software to enable detailed design. This approach required the design to operate in several software environments. An integrated system, supported by solid modeling, would enable design and assessment of a component more efficiently. A solid modeling-approach—principal feature modeling—was used to enable component-design for forging within a solid modeling environment [65, 66]; the approach enables integration of currently available 2D-based knowledge-based systems.
    Design for manufacture requires that the component form is specified in a modular form in order to enable the evaluation of the design. The component may be defined as a combination of primitive forms as is the case in “design by features;” alternatively, the primitive forms which constitute the component may be extracted and identified automatically. Unfortunately, both these approaches are currently at a stage of refinement which only allows their applications to a limited range of component forms. Principal feature modeling [67] combines the strategies of both “design by feature” and “feature recognition” to enable efficient modeling and feature manipulation; the approach was proven to be particularly efficient for the modeling of forging/machining components [65]. Designing is attended to with reference to a prescribed set of performance requirements rather than to prescribed form features. The principal features, which represent the principal geometric profiles of a component, may be defined by the designer using arbitrary geometry—a group of curves on a plane or a curved surface. The principal features which have been generated are linked, exclusively, to a set of prescribed attributes which are catalogued in a database.
    The solid model of the component may be created by geometric manipulation of a principal feature; several principal features may be defined for more complex components. Subsequent to the definition of the principal features for a particular component; local features may be added to produce the first iteration of the “performance” model of the component. The form of the additional features is generated
    by the modification of the principal geometric entities; these additional features are extracted and
    classified as individual manufacturing features. In circumstances where components cannot be modeled by
    defining principal features, the model can be created by other approaches. This will enable the prescription
    of the principal feature by the extraction of the curves on a feature plane, which is prescribed by the designer. In comparison with the “design by feature” approach, the proposed approach [67] does not require a pre-defined feature library that would constrain the flexibility of the system. Further, the difficulty and complexity of the routines for the recognition of form features are reduced since the principal features, which are recorded in the system, provide auxiliary geometric and topologic information of the component model; this simplifies the recognition of manufacturing features.



    COMPUTER-AIDED DESIGN,
    ENGINEERING, AND MANUFACTURING
    Systems Techniques And Applications
    VOLUME
    V I
    Editor
    CORNELIUS LEONDES
    Boca Raton London New York Washington, D.C.
    CRC Press
    MANUFACTURING
    SYSTEMS PROCESSES



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  • Design

    To design is either to formulate a plan for the satisfaction of a specified need or to solve a problem. If the plan results in the creation of something having a physical reality, then the product must be functional, safe, reliable, competitive, usable, manufacturable, and marketable.

    Design is an innovative and highly iterative process. It is also a decision-making
    process. Decisions sometimes have to be made with too little information, occasionally with just the right amount of information, or with an excess of partially contradictory information. Decisions are sometimes made tentatively, with the right reserved to adjust as more becomes known. The point is that the engineering designer has to be personally comfortable with a decision-making, problem-solving role.

    Design is a communication-intensive activity in which both words and pictures are used, and written and oral forms are employed. Engineers have to communicate effectively and work with people of many disciplines. These are important skills, and an engineer’s success depends on them.

    A designer’s personal resources of creativeness, communicative ability, and problemsolving skill are intertwined with knowledge of technology and first principles.
    Engineering tools (such as mathematics, statistics, computers, graphics, and languages) are combined to produce a plan that, when carried out, produces a product that is functional, safe, reliable, competitive, usable, manufacturable, and marketable, regardless of who builds it or who uses it.


    Mechanical Engineering
    McGraw−Hill Primis
    ISBN: 0−390−76487−6
    Text:
    Shigley’s Mechanical Engineering Design,
    Eighth Edition
    Budynas−Nisbett
    Shigley’s Mechanical Engineering Design,
    Eighth Edition
    Budynas−Nisbett
    McGraw -Hill




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  • Design Activities

    People have always designed things. One of the most basic characteristics of human beings is that they make a wide range of tools and other artefacts to suit their own purposes. As those purposes change, and as people reflect on the currently-available artefacts, so refinements are made to the artefacts, and sometimes
    completely new kinds of artefacts are conceived and made. The world is therefore full of tools, utensils, machines, buildings, furniture, clothes, and many other things that human beings apparently need or want in order to make their lives better. Everything around us that is not a simple untouched piece of Nature has been designed by someone. 

    In traditional craft-based societies the conception or 'designing' of artefacts is not really separate from making them; that is to say, there is usually no prior activity of drawing or modelling before the activity of making the artefact. For example, a potter will make a pot by working directly with the clay, and without first making
    any sketches or drawings of the pot. In modern industrial societies, however, the activities of designing and of making artefacts are usually quite separate. The process of making something cannot normally start before the process of designing it is complete.

    In some cases- for example, in the electronics industry- the period of designing can take many months, whereas the average period of making each individual artefact might be measured only in hours or minutes.
    Perhaps a way towards understanding this modern design activity is to begin at the end; to work backwards from the point where designing is finished and making can start. If making cannot start before designing is finished, then at least it is clear what the design process has to achieve. It has to provide a description of
    the artefact that is to be made. In this design description, almost nothing is left to the discretion of those involved in the process of making the artefact; it is specified down to the most detailed dimensions, to the kinds of surface finishes, to the materials, their colours, and so on.

    In a sense, perhaps, it does not matter how the designer works, so long as he or she produces that final description of the proposed artefact. When a client asks a designer for 'a design', that is what they want: the description. The focus of all design activities is that end-point.








    Engineering Design Methods
    Strategies for Product Design
    THIRD EDITION
    Nigel Cross
    The Open University, Mi/ton Keynes, UK
    JOHN WILEY & SONS, LTD
    Chichester- New York. Weinheim • Brisbane. Singapore. Toronto




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  • Grinding Process

    The tool in the grinding process is the grinding wheel, which is generally composed of two materials: grits and bonding agent. The hard abrasive grits (small hard particles) are affixed to rotating hub or axis in the
    general configuration of a wheel to erode material away from a workpiece as the wheel spins. Grinding wheels are formed into shape by casting and curing a slurry of bonding material with the grits. The cured wheel forms a three-dimensional matrix of grits held in place by the bonding agent. This yields a complex operation by nature, as the uneven and truly random distribution of grit surfaces of the wheel and the contact they make with the ground part are difficult at best to systematically model.

    History and Perspective
    Abrasive material removal (grinding) is one of the oldest machining technologies employed today, and has been utilized by people in the manufacturing of parts since the Stone Age (Malkin, 1989). A simplified grinding process can be thought of as milling using a “cutter” with a large number of teeth of irregular
    shape, size, and spacing (Fig. 3.2.). Each grit can be seen as a cutting tooth with varying orientation and
    sharpness. These grits are suspended in a bonding agent that holds the three-dimensional matrix of grits
    together in a form. The grinding process can vary for many reasons including: wheel sharpness, wheel microstructure, workpiece material variation, loading of workpiece material on the wheel, and other phenomena that contribute to the changing nature of the grinding process. (Loading is the phenomena of the ground material becoming attached to or embedded onto the surface of the grinding wheel. This effect begins with the material filling in the voids around the grits, and if permitted to continue, the material can eventually cover the grits. This occurs more commonly with softer materials.)
    The variable nature of the grinding process has been linked to the physical descriptions of the actions between the grits with the workpiece. Grinding is a complex operation that can be seen as three separate and concurrent process actions: (1) cutting, (2) plowing, and (3) rubbing (Hahn and Lindsay, 1971; Samuels, 1978; Salmon, 1992). The cutting action produces tangential forces that are related to material specific energy in the generation of chips. Although a small part of this energy is transferred to the chip as kinetic energy, the majority of the material specific energy is dispersed in several other ways including: friction, heat conduction, radiation, surface formation, residual stresses, etc. The second grinding process action is plowing. In plowing material is plastically deformed around the sides of dull grits, leading to ruts and uneven surfaces. No material is removed with plowing. Rubbing occurs as material loads onto the wheel or if the grits are exceptionally dull (causing attritious flat wear). Rubbing is a high friction condition with essentially no material removal and is the most detrimental of the three grinding actions.
    The generated frictional heat (that can yield “burning”) from rubbing is dissipated into both the ground
    part and the wheel. If coolant is not used, thermal damage may occur. Damage created from excess forces
    or from plowing ruts can be hidden by the migration of material during rubbing, only to result in premature failure of the part in use. In general, all three grinding process actions occur simultaneously in varying distributions depending on the grinding wheel, material, and operating conditions. As stated previously grinding is stochastic by its nature, making detailed process analysis, modeling, and control difficult. This is also brought out by the variety and complexity of grinding models presented later.



    COMPUTER-AIDED DESIGN,
    ENGINEERING, AND MANUFACTURING
    Systems Techniques And Applications
    VOLUME
    V I
    Editor
    CORNELIUS LEONDES
    Boca Raton London New York Washington, D.C.
    CRC Press
    MANUFACTURING
    SYSTEMS PROCESSE

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  • LATHE

    INTRODUCTION
    The lathe is father of all machine tools; in early days it was equipped with a fixed tool rest and was used for woodworking. In operation, the lathe holds the job between two rigid supports called centres or by some chuck or face plate screwed to the nose or and of the spindle.
    Function of lathe
    The main function of lathe is to remove metal from a piece of work to give it the required shape and size. This is accomplished by holding the work securely and rigidly on the machine and then turning it against cutting tool, which will remove, metal from the work in the form of chips.
    Types of lathes
    Lathes of various designs and constructions have been developed to suit the various conditions of metal machining. But all of them employ the same fundamental principle of operation and perform the same function. The lathes are classified as follow.
    1. Speed lathe
     Wood working
     Centering
     Polishing
     Spinning
    2.Engin lathe
     Belt drive
     Individual motor drive
     Gear head lathe
    3.Bench lathe
    4.Tool room lathe
    5.Capstan and turret lathe
    6.Special purpose
    Wheel lathe
     Gap bed lathe
     T-lathe
     Duplicating lathe
    7.Automatic lathe

    The speed lathe

     The speed lathe, in construction and operation, is the simplest of all types of lathes. It consists of a bed, a headstock, and a tailstock and tool post mounted on an adjustable slide. There is no feed box, lead screw or conventional type carriage. The tool is mounted on the adjustable slide and is fed into work purely by hand control. This characteristic of the lathe enables the designer to give high spindle speeds, which is usually, range from 1200 to 3600 r.p.m. As the tool is controlled by hand, the depth of cut and thickness of chip is very small.
     Light cut and high speed necessitate the use of this type of machine where cutting force is minimum such as in wood working, spinning, centering, polishing, etc.
    The engine lathe or center lathe
    This lathe is most important member of lathe family and is most widely used. Similar to the speed lathe, the engine lathe has got all the basic parts, e.g. bed, headstock, and tailstock. But the headstock of an engine
    lathe is much more robust in construction

    and it contains additional mechanism of driving the lathe spindle at multiple speeds.
    The engine lathe that can feed the cutting tool both in cross and longitudinal direction with reference to the lathe axis with help of a carriage feed rod and lead screw.

    The bench lathe

    This is a small lathe usually mounted on bench. It has practically all the parts of an engine lathe or speed lathe and it performs almost all the operations, its only difference being in size this is used for small and precision work.

    The tool room lathe

    A tool room lathe having features similar to an engine lathe is much more accurately built and has a wider range of spindle speeds ranging from a very low to a quite high speed up to 2500 r.p.m.This lathe is mainly used for precision work on tools, dies, and gauges and in machining work where accuracy is needed


    Numerically Controlled Lathes
    In these lathes the path of the tools to produce the desired shape, along with other auxiliary functions like speed/feed changes, turret indexing, tail stock positioning, coolant supply, etc., are controlled by pre-programmed numerical data input in the form of punched tape to an electronic control system. One of the latest developments is the computer control of NC machines which is popularly called the computer numerical control (CNC). With the concept of numerical control, the configuration of lathes has undergone radical changes with features like slant bed design infinitely variable seed headstock, antifriction lead screws, auto-tool changing, etc. Numerically controlled lathes are available in various versions. The control system employed on NC lathes enable straight cut and continuous path control.

    The cutting tools on a CN C turning center are usually mounted in a device called a turret. The turret is then mounted on a heavy cross slide and carriage similar to that of a conventional lathe. The turret can be indexed very rapidly to automatically change to the next tool.
    A number of smaller CNC lathes have also come to market in recent years to compete with the turret lathes and automatic screw machines.





    fr:NETTUR TECHNICAL TRAINING FOUNDATION



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  • Machine tool

    Machine tool is a machine which is moved by the manpower (Electrical, mechanical, hydraulic and pneumatic) are used to create a shape, size, and precision or accuracy (according to the design) with remove metal from a coupon or a workpiece or specimen (Workspieces) in the form of anger. Machine tools are factory equipment
    to produce the machines, instruments, tools and tooling for the entire needs. It could be argued that the machine tool is the mother of all machine.

    Most machine tools perform four functions, namely:
    a. Keeping a job
    b. Maintain cutting tools
    c. Moving one or more of the rotational movement or motion back and forth
    d. Provides a feed movement to rotational movement or back and forth

    3.2 Classification
    Machine tools can be classified in several ways.
    A. Based on the scope of application, machine tools can be classified into:
    a. General use machine tools
    Machine tools for general use or a universal machine tools widely used to make various specimens with broad coverage includes only the pieces can make it, small lot production, and for repairs. Machine tools used for a particular scope of work is widely known with the name of multi-purpose machine tools (multipurpose). That
    included into the machine tools for general use is plain turning lathes, turret lathes, milling machines, drilling machines, grinding machine and so on.

    b. Single-use machine tools
    This type of machine tools used to create a certain type machining operations, eg, broaching, thread cutting, gear shaping and hobbing machine, machine for machining pistons, crankshaft, for turning the camshaft and cam Contours on camshafts and so on.

    c. Machine tools for limited use
    Machine tools capable of this type for an operation on a narrow various kinds of workpieces, for example, automatic cutting off machines.

    d. Machine tool production
    Machine tools of this type are widely used manufacturing lots production, mass production, high-production features and stiffness. That included in this type of machine is a multi-tool lathes, single and multi-spindle automatic, semi-automatic lathe, plunge-cut cylindrical grinder, centreless, planer-type milling machine, thread rolling machine for tap production, numerically controlled machine tool, and so on. 

    e. Specialized machine tools
    This type of machine tools used to create a form similar but different sizes. This type of machine tools also perform the process machining multiple surfaces on different areas. The advantage of these machine tools are able to do change from one job to another job. This can be done because install the head where there is an additional angle can be changed in the horizontal plane or vertical plane or vice versa. This machine widely used to produce large lot.

    f. Machine specific tooling
    This type of machine tools in design and manufactured individually with the intent to establish something on the machining operation the particular and the particular workpieces as well. Machine type These include machines for sharpening round whorls dies, mengerinda The slanted edges around the whorls dies, marking around the whorls type stalk dies and tooling, for whorls by die tap, for grinding flute on tap and reamer, tap chamfer, flutes on a twist drill, and so on. This type of machine is widely used in the production-lot production
    large as well as mass.(Nafsan Upara)




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  • PLASTICITY - STRESS–STRAIN RELATIONSHIP - ELASTIC LIMIT

    PLASTICITY is that state of matter where permanent  deformations or strains may occur without fracture. A material is plastic if the smallest load increment produces a permanent
    deformation. A perfectly plastic material is nonelastic and has no ultimate strength in the ordinary meaning of that term. Lead is a plastic material. A prism tested in compression will deform permanently under a small load and will continue to deform as the load is increased, until it flattens to a thin sheet. Wrought iron and steel are plastic when stressed beyond the elastic limit in compression. When stressed beyond the elastic limit in tension,
    they are partly elastic and partly plastic, the degree of plasticity increasing as the ultimate strength is approached.

    STRESS–STRAIN RELATIONSHIP gives the relation between unit stress and unit strain when plotted on a stress–strain diagram in which the ordinate represents unit stress and the abscissa represents unit strain. Figure 5 shows a typical tension stress–strain curve for medium steel. The form of the curve obtained will vary according to the material, and the curve for compression will be different from the one for tension. For some materials like
    cast iron, concrete, and timber, no part of the curve is a straight line.

    PROPORTIONAL LIMIT is that unit stress at which unit strain begins to increase at a faster rate than unit stress. It can also be thought of as the greatest stress that a material can stand without deviating from Hooke’s law. It is determined by noting on a stress–strain diagram the unit stress at which the curve departs from a straight line.

    ELASTIC LIMIT is the least stress that will cause permanent strain, that is, the maximum unit stress to which a material may be subjected and still be able to return to its original form upon removal of the stress.

    JOHNSON’S APPARENT ELASTIC LIMIT. In view of the difficulty of determining precisely for some materials the proportional limit, J. B. Johnson proposed as the ‘‘apparent elastic limit’’ the point on the stress–strain diagram at which the rate of strain is 50% greater than at the origin. It is determined by drawing OA (Fig. 5) with a slope with respect to the vertical axis 50% greater than the straight-line part of the curve; the unit stress at which the line O A which is parallel to OA is tangent to the curve (point B, Fig. 5) is
    the apparent elastic limit. 

    YIELD POINT is the lowest stress at which strain increases without increase in stress. Only a few materials exhibit a true yield point. For other materials the term is sometimes used as synonymous with yield strength. 

    YIELD STRENGTH is the unit stress at which a material exhibits a specified permanent deformation
    or state. It is a measure of the useful limit of materials, particularly of those whose stress–strain curve in the region of yield is smooth and gradually curved.

    ULTIMATE STRENGTH is the highest unit stress a material can sustain in tension, compression, or shear before rupturing.

    RUPTURE STRENGTH, OR BREAKING STRENGTH, is the unit stress at which a material breaks
    or ruptures. It is observed in tests on steel to be slightly less than the ultimate strength because of a large reduction in area before rupture.

    MODULUS OF ELASTICITY (Young’s modulus) in tension and compression is the rate of change of unit stress with respect to unit strain for the condition of uniaxial stress within the proportional limit. For most materials the modulus of elasticity is the same for tension and compression.

    MODULUS OF RIGIDITY (modulus of elasticity in shear) is the rate of change of unit shear stress with respect to unit shear strain for the condition of pure shear within the proportional limit. For metals it is equal to approximately 0.4 of the modulus of elasticity.

    MECHANICAL DESIGN
    Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
    Edited by Myer Kutz
    Copyright  2006 by John Wiley & Sons, Inc.



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  • The casting design

    In the casting design, factors to consider are:
    1. The function of the casting,
    2. The ability of the casting,
    3. Strength casting,
    4. Ease of production,
    5. Considerations for safety
    6. Economies in production.
    In order to meet this requirement, we must have a thorough knowledge of production methods including pattern making, molding, core making, melting and flow, etc.
    The best design will be achieved only when one is able to make the right choice from a variety of methods available. However, some rules for designing castings are given below to serve as a guide:
    1. Sharp corners and often use the fillet should be avoided to avoid stress concentrations.
    2. All parts must be designed in a casting of uniform thickness, as far as possible. If, however, the variation is unavoidable, it should be done gradually.
    3. A sudden change from the very thick to very thin sections should always be avoided.
    4. Casting should be designed as simple as possible, but with a good appearance.
    5. Large flat surface on the casting should be avoided because it is difficult to obtain the correct surface on large castings.
    6. In designing the casting, various allowances should be provided in making the pattern.
    7. The ability to withstand pressure of casting contraction of some members can be enhanced by providing for example a curved shape, arms, pulleys and wheels.
    8. The rigid members such as webs and ribs are used in the casting must be at least possible amount, because it can cause various defects such as hot water and shrinkage, etc.
    9. Casting should be designed in such a way that would require a simple pattern and the mold easier.
    10. In order to design cores for casting, consideration should be given to provide them adequate support in the mold.
    11. Deep and narrow pockets in the casting should always be avoided to reduce cleanup costs.
    12. The use of metal inserts in the casting must be kept at a minimum.
    13. Signs such as names or numbers, etc., should not be given on vertical surfaces because they provide a barrier in the withdrawal pattern.
    14. A tolerance of ± 1.6 mm in the casting of small (under 300 mm) must be provided. In terms of accuracy over the desired dimensions, tolerance ± 0.8 mm can be given.





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  • Classes and Characteristics of Composite Materials

    There is no universally accepted definition of a composite material. For the purpose of this work, we consider a composite to be a material consisting of two or more distinct phases, bonded together.

    Solid materials can be divided into four categories: polymers, metals, ceramics, and carbon, which we consider as a separate class because of its unique characteristics. We find both reinforcements and matrix  materials in all four categories. This gives us the ability to create a limitless number of new material systems with unique properties that cannot be obtained with any single monolithic material. Table 1 shows the types of material combinations now in use.

    Composites are usually classified by the type of material used for the matrix. The four primary categories of composites are polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs), and carbon/carbon composites (CCCs). At this time, PMCs are the most widely used class of composites. However, there are important applications of the other types, which are indicative of their great potential in mechanical engineering applications.

    Figure 1 shows the main types of reinforcements used in composite materials: aligned continuous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numerous forms of fibrous  architectures produced by textile technology, such as fabrics and braids. Increasingly, designers are using hybrid composites that combine different types of reinforcements to achieve more efficiency and to reduce cost.

    A common way to represent fiber-reinforced composites is to show the fiber and matrix separated by a slash. For example, carbon fiber-reinforced epoxy is typically written ‘‘carbon/ epoxy,’’ or, ‘‘C/Ep.’’ We represent particle reinforcements by enclosing them in parentheses followed by ‘‘p’’; thus, silicon carbide (SiC) particle-reinforced aluminum appears as ‘‘(SiC)p/ Al.’’

    Composites are strongly heterogeneous materials; that is, the properties of a composite vary considerably from point to point in the material, depending on which material phase the point is located in. Monolithic ceramics and metallic alloys are usually considered to be homogeneous materials, to a first approximation.

    Many artificial composites, especially those reinforced with fibers, are anisotropic, which means their properties vary with direction (the properties of isotropic materials are the same in every direction). This is a characteristic they share with a widely used natural fibrous composite, wood. As for wood, when structures made from artificial fibrous composites are required to carry load in more than one direction, they are used in laminated form. 

    Many fiber-reinforced composites, especially PMCs, MMCs, and CCCs, do not display plastic behavior as metals do, which makes them more sensitive to stress concentrations.  However, the absence of plastic deformation does not mean that composites are brittle materials like monolithic ceramics. The heterogeneous nature of composites results in complex failure mechanisms that impart toughness. Fiber-reinforced materials have been found to produce durable, reliable structural components in countless applications. The unique characteristics of composite materials, especially anisotropy, require the use of special design methods.



    Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
    Edited by Myer Kutz
    Copyright  2006 by John Wiley & Sons, Inc.

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  • Metal Casting Processes

    Casting is making components in a way pour the melted material into the mold. Material herein may the form of metal and non-metal. To melt the ingredients necessary furnace (cupola kitchen). Furnace is a kitchen or a place equipped with a heater (heating). The solid material melted until temperature melting point and can be added to the mixture of materials such as chrome, silicon, titanium, aluminum and other materials in order to become more good. Materials that are liquid can be poured into molds. Molds for casting can be made with sand or
    metal. For components that are complex and numerous usually use sand mold, while the components that form simple and can use any mass-produced metal molds. In making molds that need to be considered is the porosity and tolerance for sringkage (depreciation) after casting. porosity the higher the better mold to release the gases trapped inside the mold.






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